A portable telephone, for example, as shown in FIG. 8, has a flat casing 3 comprising the combination of a front case 31 and a rear case 32, and is provided with an antenna 33 on a head portion of the casing 3.
Disposed on a surface of the front case 31 are a key input device 34 having a plurality of manual keys and a LCD (Liquid Crystal Display) 35. Provided respectively on a upper and a lower end portions of the surface are a speech receiving portion 36 incorporating a speaker and a speech delivery portion 37 incorporating a microphone. When the manual keys of the key input device 34 are manipulated, information such as characters, etc. input by the manipulation is displayed on the LCD 35. When a voice call or an electronic mail is received, a telephone number or a name of a caller or a sender is displayed on the LCD 35. In a call waiting state, information like date or a present time is displayed on the LCD.
The casing 3 has a key light device (not shown) for lighting a manipulation surface of the key input device 34 and a backlight (not shown) for lighting a screen surface of the LCD 35 incorporated therein. When the manual key of the key input device 34 is manipulated or when a voice call is received or when an electronic mail is received, the key light device and the backlight are turned on for a predetermined period of time, e.g., 10 seconds. The key light device and the backlight are turned on for a predetermined period of time only when the manual key is manipulated, or the voice call is received, or the electronic mail is received, so that power consumption can be saved.
However, with the conventional portable telephones, when a user keeps a handset over his ear while calling, he does not look at a screen of the LCD 35. Display operation of the LCD 35, though, is always actuated to waste useless power. This entails the problem of shortening not only the battery life of the secondary battery but also standby time and continuous talktime.
Particularly, in recent years, portable telephones comprising a TFT liquid crystal display instead of a conventional STN liquid crystal display have appeared on the market. With the portable telephones of this type having a TFT LCD, there arises the distinctive problem of shorter battery life, shorter standby time, and shorter continuous talktime since the TFT LCD is greater than STN LCD in power consumption.